1. Field of Invention
This invention relates to an oscillation circuit and an electronic circuit, as well as to a semiconductor device, a timepiece, and an electronic device equipped with the same.
2. Description of Related Art
Many of today's wristwatches and other portable electronic devices use a disposable battery as a main power source and also have a rechargeable secondary battery, to drive the electronic circuitry thereof. The electronic circuitry in such devices often creates a reference clock signal from the oscillation frequency fs of an oscillation circuit.
The main circuit components of this oscillation circuit are usually formed on a semiconductor board and a crystal oscillator that is provided in a separate location from this semiconductor board is connected thereto by an input-output terminal. Therefore, an electrostatic protective circuit is provided on an input-output terminal side of these main circuit components, to protect those main circuit components from any surge voltage that may invade from the outside through the input-output terminals.
This causes a problem in that, if the load on the electronic circuitry should increase, causing fluctuations in the voltage V.sub.SS of the main power source, these fluctuations will induce fluctuations in the parasitic capacitance of the electrostatic protective circuit and, as a result, the oscillation frequency of the oscillation circuit will also fluctuate.
Therefore, the electrostatic protective circuit comprises a diode connected between a signal line and the ground side and a diode connected between the signal line and the main power source V.sub.SS side, and it is designed to protect the internal circuitry by selectively diverting any surge voltage that invades this signal line toward either the ground side or the power-voltage side, depending on the polarity of the surge voltage.
Unfortunately, the parasitic capacitance is created at the pn junctions of the diodes of this electrostatic protective circuit. The magnitude of this parasitic capacitance varies with fluctuations in the power voltage V.sub.SS caused by factors such as fluctuations in load.
However, if the parasitic capacitance varies in this manner, the oscillation constant of the oscillation circuit will also vary, changing the oscillation frequency itself. As a result, the reference clock signal frequency will vary, causing a problem in that the operation of the various components of the electronic circuitry will be adversely affected.
This causes a particular problem in an electronic circuit that uses the oscillating output of the oscillation circuit as a reference clock signal for a timepiece, such as an electronic circuit for a wristwatch, because if the oscillation frequency varies, the reliable operation of the timepiece itself will be lost.